meiosis - ms jmo's biology hivespacemsjoconnor.weebly.com/.../1.2_celldivision_meiosis.pdf ·...
TRANSCRIPT
Meiosis The sexy shuffling machine
LO:
Describe the events of meiosis
Explain how meiosis creates ‘uniqueness’
Compare & contrast mitosis & meiosis
http://www.youtube.com/watch?v=kVMb4Js99tA
• DNA replication occurs in interphase followed by two cycles of division (cytokinesis).
• 1 diploid (di = double DNA) makes 4 haploid daughters (ha = half)
• Daughter cells are sex cells (gametes)
• Genes are shuffled between homologous chromosomes – daughters are genetically unique
• Homologous chromosomes are seperated randomly
What does meiosis make?
http://cellsalive.com/meiosis.htm
Meiosis Intro
http://www.youtube.com/watch?v=kVMb4Js99tA
Meiosis - a 2-step process
• Meiosis 1 is the reduction division (prophase 1, metaphase 1, anaphase 1 and telophase 1)
• Meiosis 2 resembles mitosis (prophase 2, metaphase 2, anaphase 2 and telophase 2)
Meiosis 1: Prophase 1
• Chromosomes condense
• Homologous chromosomes pair up
• CROSSING OVER
• Spindle fibres grow from poles equator
• Nuclear membrane about to break down
Meiosis 1: Metaphase 1
• Pairs of line up on the equator
• Orientation is random
• Spindle fibres attach to each of a pair of homologous chromosomes
Meiosis 1: Anaphase 1
Homologues are pulled apart, ½ chromosome #
Move to opposite poles
Cell membrane soon to be pulled inwards (cytokinesis)
RESHUFFLING 2n combinations Humans: 223 = >8 million
combinations
Meiosis 2: Prophase 2
• Cells are haploid • Chromosomes do not
decondense at the end of meiosis 1
• Each chromosome has still two chromatids
• New spindle microtubules form
Meiosis 2: Metaphase 2
• The chromosomes line up on the spindle equator independently
Meiosis 2: Anaphase 2
• The sister chromatids randomly separate on the spindle
• Each cell will receive a copy of each chromosome
• The genes on the different chromosomes are recombined (shuffled)
Meiosis 2: Telophase 2
• Four genetically unique haploid sex cells are produced
• New nuclear membranes form
Genetic Variation: Crossing over
• Prophase 1 : homologous pairs link & exchange genetic material
• The genes on each chromosome -not identical ,may be alleles
• Alleles = different versions of a gene
• Ear shape gene – 2 alleles: ear lobe allele and no ear lobe allele
Genetic Variation: Law of Independent Assortment
• Metaphase I & II: Spindle microtubules randomly attach to chromosomes or chromatids on the metaphase plate
• Independent/Random assortment
• Combined with random mating , infinite variety of individuals is possible.
This means that when they fuse together during fertilisation the zygote has the same number of chromosomes as the
parents’ cells
Why is it important that a cell produced my meiosis has half the
DNA of the parent cell?
Formation of gametes
46 46
23 23
How many chromosomes here?
Creating Uniqueness (Genetic variation)
• Crossing over of chromosome arms (prophase I)– creates new combinations of genes
• Independent assortment of chromosomes (metaphase I)
• Orientation of chromosomes along the equator in metaphase 2 is random
• Random mating
The ‘crossing over’ of some genes (prophase I) Means: 1. The gametes produced carry different combinations of
genes 2. They do not have the same combination as the parent
cell 3. They do not have the same combination as each other
GAMETES PARENT
CELL
Independent assortment of chromosomes (Metaphase I & II)
Means: 1. The chromosome received by a gamete from each
homologous pair is random. 2. Any combination of chromosomes (as long as one of
each is received) is possible in a gamete 3. Meiosis means we are infinitely unique!
GAMETES PARENT
CELL
Creating Uniqueness (Genetic variation)
Your so unique!
• 23 pairs chromosomes = 223 possible combinations for a single gamete
• Meiosis promotes genetic variation within a species and maintains the correct chromosome number
Steps of Meiosis http://www.cellsalive.com/meiosis.htm
Why does the line go
up/down when it does?
Why is it a flat-line all the
way through?
Why up once and down
twice? What phases are
each of these happening in?
Why does genetic info go
down?
http://www.youtube.com/watch?v=iCL6d0OwKt8
Feature Mitosis Meiosis
Definition Cell division resulting in
two daughter cells each
having identical number
and type of chromosomes
as the parent nucleus.
A reduction division
resulting in four daughter
cells (gametes) each with
unique genetic
information and half the
number of chromosomes
of the parent cell
Occurs in (cell type) Totally everywhere Sex organs
Number of divisions 1 2
Number of daughter cells produced
2 4
Genetic Composition Same as parent Unique
Creates (type of cell) Any (non gamete) Gametes only
Genetically / Type of Reproduction
Asexual Sexual
Function / Importance in Body
Growth & repair Reproduction
Human Chromosome number
46 23